LOS ANGELES (AP) — “You’re Next” is a nasty little slasher film that starts poorly but gets better once most of the cast has been butchered.

Indie film figures Joe Swanberg and Ti West play two attendees at a party where four siblings and their significant others are celebrating their parents’ 35th wedding anniversary. Most tolerable among this largely annoying crew are Crispian (A.J. Bowen), a college prof, and his Australian girlfriend Erin (Sharni Vinson), but that’s not paying the two very high praise. The irritation factor grows substantially after the first slaying at this remote Tudor mansion, when half the female cast seems to be competing to shriek the longest.

An unknown number of men, wearing animal masks and wielding crossbows (why not guns?), are stalking the family from without and within the house; since director Adam Wingard and screenwriter Simon Barrett have given themselves so many characters to kill, they start off with a few quick killings in which the victims are behaving so stupidly they’re practically asking to die.

Most frustrating during the film’s first half is that only one among the 10 characters, Erin, has anything approaching a self-preservation instinct. While others scream or stand around dumbly, she hustles off to lock windows and gather weapons. While the mask-wearing villains have a hard time delivering the kind of novel slayings horror fans demand, Vinson musters the ferocity to compensate — the moment she meat-tenderizes an attacker’s skull, the movie starts to turn fun.

In the absence of sympathetic characters, a little humor would have gone a long way here. But aside from a near-miss sex scene in a bed shared by a corpse, there’s practically none on hand. Only when the reasons for the attack become clear does the movie find its feet, but “You’re Next” ends on a high enough note that buzz on the way out of the theater should work in its favor.